Reflow soldering is a process for soldering together pretinned electrical circuit parts. Reflow soldering is used extensively in the manufacture and repair of printed circuit boards. Some typical applications are soldering of ribbon cable to printed circuit conductors, and soldering of the leads of integrated circuit chips to printed circuit conductors.
One version of a reflow soldering process is called parallel gap reflow soldering. This process involves passing electrical current through the parts to be soldered to form a single joint. Two other process versions are called single point pulse heated reflow soldering, and multiple lead reflow soldering; each of these process versions involves passing electrical current through a reflow soldering tip (a "thermode") that is pressed against the pretinned parts so as to heat them by thermal conduction.
The multiple lead reflow soldering version reduces assembly time because it makes a number of solder joints simultaneously.
To accomplish multiple lead reflow soldering reliably in a production operation involves meeting numerous requirements. Some of these requirements are to provide for heating each of the multiple joints to the same temperature for the same amount of time. To cause the solder to flow, the thermode must supply enough heat to raise the solder temperature to about 600.degree. F., and to make a reliable solder joint, such high temperature must be maintained for a time in the range of about 2 to 10 seconds. Another requirement is to ensure that joint-to-joint voltage gradients are kept sufficiently small so as to avoid damage to associated circuitry.
Meeting these requirements has proven to be a difficult problem in multiple lead reflow soldering. One reason why the problem is so difficult relates to variable heat sinking. That is, each of the two joints that are at the opposite end of a line of joints can conduct heat into the printed circuit board in a different way from the way the interior joints do. Side-by-side joints in the line can distribute heat to one another, whereas an end joint on one of its sides simply conducts heat into the board. Another reason the problem is so difficult relates to various manufacturing tolerances in dimensions. An ideal circumstance would be for the tops of all the pretinned parts of a group to be positioned along a straight horizontal line, so that a thermode could be brought down to make contact with all of them at the same instant, and bear on all of them with the same pressure while supplying heat to them for the required time at temperature. Such a circumstance is purely ideal; the reality in a production operation is that the thickness of the pretinned layer of solder varies in an amount significant to the problem involved here. Other components such as the board, the printed circuit conductors, the leads projecting from integrated circuits are also subject to dimensional variations, and any such dimensional variations can aggravate this problem.
A great deal of effort has been expended on designing thermodes in an effort to meet these requirements, and overcome these problems. Some thermode designs are categorized as bar heaters; others are categorized as fold-up heaters.
A bar heater, in a rudimentary design, comprises a relatively small diameter, electrically insulated wire formed into an upwardly open U shape. That is, it has a central pressing portion and spaced-apart attaching portions. Each of the attaching portions is affixed to a respective one of two terminal plates that form part of the circuit for the heating current. To minimize I.sup.2 R loss in the terminal plates, they are generally made of copper and are sized to provide a substantial cross-sectional area through which the heating current flows. In such a rudimentary bar heater, each terminal plate has a generally L-shaped configuration. The terminal plates are positioned side-by-side, so that together they present an inverted T-shaped profile. Each terminal plate has an electrically conductive surface for engaging a corresponding electrically conductive surface defined in a movable mounting support. Mounting means provide for pressing the engaging surfaces together very tightly. A high pressure engagement is necessary to minimize the resistance caused by the surface contacting, and thereby minimize thermal loss. The heating current is in the order of about 100 amps. The wire that serves as the heating element can be made of Nickel-Chrome and can have a thin oxide coating to provide electrical insulation. Such a heating element can be heated by the heating current to a temperature in the order of about 500.degree. C. (932.degree. F.). The electrically insulating oxide coating serves as a means to protect against joint-to-joint voltage gradients. This protection can be very important to ensure that voltage-sensitive integrated circuits on the printed circuit board are not damaged by the operation of soldering the joints.
Such a rudimentary bar heater thermode does not provide any means for dealing with the problem discussed above with reference to dimensional variations causing the joint tops to have varying heights. Furthermore, there are numerous attributes of such a rudimentary bar heater thermode that are not preferable. To begin with, such a rudimentary design has its own tolerance problem. This problem is attributable to the curved portions at opposite ends of the pressing portion that join the pressing portion to the attaching portions. Taking into account such tolerances requires a design center length for the pressing portion such that it will be longer than the linear space from one end joint to the opposite end joint. The round cross section of the wire is not a preferable shape for a thermode heating element; it provides for only a line contact rather than an area contact when pressed down against the multiple leads. The electrical insulation is not preferable because it reduces thermal efficiency. To amplify further on the need for such electrical insulation in such a rudimentary, as well as other bar heater designs, the high heating current causes a voltage gradient along the length of the pressing portion.
As a result of the great deal of effort that has been expended in efforts in this field, improved bar heater thermodes have been designed having more preferred features. So far as is known, no such improved design has provided any means for dealing with the problem discussed above the reference to dimensional variations causing the joint tops to have varying heights. As to improved and more preferred features, bar heater thermodes have been designed to include an essentially flat bottomed heating element so as to provide the more preferable area contact rather the line contact provided by the wire type described above. Another improvement has been made in an effort to deal with the problem associated with the extra heat sinking associated with opposite end joints in comparison with interior joints. In particular, there are flat-bottom bar heaters that have a varying cross section. Near each opposite end, the area of such an improved bar heater's cross section is small in comparison with the area of a cross section taken elsewhere. Inasmuch as resistance of such a conductive element is a function of cross section, it will be appreciated that the current flowing through such an improved bar heater passes through a higher resistance portion of the heater, proceeds through a lower resistance portion, and then passes through another higher resistance portion before proceeding out to the terminal plate. Inasmuch as the heat generated is proportional to the I.sup.2 R dissipation, it will be appreciated that higher temperatures can be produced at each opposite end by taking this approach of tailoring the resistance of the heating element along its length. However, there are factors that limit how far one can proceed with this approach. By reducing the cross section close to each opposite end, one sacrifices strength and rigidity.
Turning now to thermodes in the fold up category, such thermodes incorporate numerous preferred features. An important one of these preferred features relates to the direction in which current flows relative to the pressing portion. In contrast to a the bar type heater in which the current flows along the length of the pressing portion, the fold up type thermode has current flow perpendicularly to the pressing portion. The current can be visualized as a wide stream sheeting down, changing direction in passing across the flat bottom, and flowing back as a wide stream. Thus, there is little if any voltage gradient along the horizontally extending length of the pressing portion. Instead, the voltage gradient is developed along the vertically extending portions of the fold up heating element. As a result, there is no need for electrical insulation on the fold up heating element. It can be used safely with even highly sensitive integrated circuits connected to the joints to be soldered. The elimination of need for electrical insulation provides for improved thermal efficiency. Notwithstanding the improvements involved in the fold up thermode, problems have remained unsolved. So far as is known, every prior art thermode in the fold up category, like every prior art thermode in the bar type category, lacks any means for dealing with the problem discussed above with reference to dimensional variations causing the joint tops to have varying heights. Like thermodes of the bar type, thermodes of the fold up type have been rigidly mounted to a movable support in a way that does not admit of accommodating to the circumstance of the joints to be soldered having varying height tops. Further, so far as is known, every prior art thermode in the fold up category, like every prior art thermode in the bar type category, lacks a means that, without sacrificing strength and rigidity, solves the problem of higher heat sinking by opposite end joints in a group of joints to be soldered.
In view of the foregoing, it will be appreciated that there exists a need for an invention to solve the foregoing problems.